The present invention relates to a driving circuit for N-channel power MOS transistors of push-pull stages.
As is known, in push-pull stages made with N-channel MOS technology, the upper device requires a gate voltage higher than the supply in order to reach a high-conductivity state. For this purpose, a bootstrap circuit is employed, which allows to obtain the required voltage.
A known circuit of this kind is illustrated, for clarity, in FIG. 1 showing a MOS power transistor 1, which constitutes the lower device, a MOS power transistor 2, which constitutes the upper device, an input 3 for the signal and a load 4 connected with a terminal to the common point of the transistors 1 and 2. The circuit furthermore comprises a bootstrap capacitor 5 connected with its terminals between the source S of the transistor 2 and the gate G of the latter through the circuit comprising the transistor 6, the current source 7, the diode 8, and the MOS transistor 9. Furthermore, a diode 10 is provided, being connected with its anode to the supply voltage V.sub.CC and with its cathode to the rest of the circuit.
In DC operation, the gate of the transistor 2 is connected to the supply voltage through the bootstrap circuit. Consequently the load can be fed even if at a voltage lower than the (positive) supply. As an example, if the supply voltage V.sub.CC is 30 V, on the load it is possible to obtain a voltage of 20 V, given as a first approximation by the difference between the supply voltage and the gate to source drop (V.sub.GS). In such conditions, the power transistor operates open and dissipates high power, through it is capable of feeding the load. In AC operation, however, it is disadvantageous to have the bootstrap system apply to the gate of the power transistor, with respect to the source, a voltage equal to the supply voltage. Such a voltage value is too high, since, in order to have a good driving, it is necessary to apply a voltage V.sub.GS comprised between approximately 10 and 14 V, while voltages higher than 20 V can be dangerous for the MOS power transistor itself.
In order to solve the above described AC operation problem, it is possible to supply the bootstrap circuit at a lower voltage, as an example 12 V. Such a solution is shown, as an example, in FIG. 2, in which the same elements of FIG. 1 have been designated with the same reference numeral. In particular, as can be noted, the circuit of FIG. 2 is different from the one of FIG. 1 only for the fact that the anode of the diode 10 is no longer connected to the voltage supply V.sub.CC, but is fixed to a suitable lower constant voltage (e.g. 12 V).
The configuration shown in FIG. 2 does indeed solve the AC operation problem, by virtue of the connection of the gate of the transistor 2 to a lower voltage, but it is no longer capable of giving power to the DC lead. Indeed, the MOS transistor 2, in order to conduct current, needs a voltage drop V.sub.GS of approximately 10 V. Since, during DC operation, the gate circuit is fed at low voltage (in the example shown, at 12 V), it has not a voltage sufficient to supply the load, and the circuit shown is not capable of operating in direct current.
In order to solve the problems presented by the circuits shown in FIGS. 1 and 2, that is to say, in order to obtain a circuit capable of reliable DC and AC operating, a solution such as the one shown in FIG. 3 has been studied by the Applicants. Such a circuit (in which the elements equal to the preceding circuits have been indicated with the same reference numerals) is different from the preceding ones due to the fact that between the bootstrap circuit and the supply voltage V.sub.CC two zener diodes 11' and 11" are arranged which have for example a break down voltage at 7 V and are series coupled in order to hold, together, 14 V. Such a circuit is capable of operating reliably both in the DC mode, in which the gate circuit of the transistor 2 is connected to the supply voltage V.sub.CC by means of the two diodes, and is therefore capable of applying sufficient power to the load, and in the AC mode, since, when the device goes in bootstrap, the diodes break at 14 V, so that the drop V.sub.GS remains locked at this value.
Such a device, however, has the disadvantage of absorbing high power for its operation, without this power being usable or transferred to the load. Indeed, at every operating cycle, the capacitor 5 charges to the supply voltage V.sub.CC and then, during the bootstrap phase, discharges the excess voltage on the two zener diodes 11' and 11" which lock the voltage to the preset value. Consequently, at each cycle power is taken for charging the capacitor, which energy is then dissipated in the discharge of the capacitor through the zeners 11' and 11". Consequently, the circuit of FIG. 3, though it solves the problem of adequately supplying the load in the DC mode and of ensuring the AC operation, has the disadvantage of being too dissipative, which causes its use to be impossible or anyhow disadvantageous in most cases.